U.S. Pat. No. 4,679,016 entitled "Interchangeable Mechanism for Molded Case Circuit Breaker" describes a thermal and magnetic trip unit wherein a pair of separable contacts are held in electric circuit with each other against the opening bias of a pair of powerful operating springs by means of a latch assembly. The circuit breaker becomes unlatched by contact between the trip unit and the pivotally-mounted trip bar to allow the operating springs to drive the contacts to their open position. When such circuit breakers are used within industrial applications, the bi-metal employed within the thermal and magnetic trip unit is designed to generate sufficient trip force to overcome the circuit breaker latch to release the circuit breaker operating springs. In many applications, the bi-metal trip force is inversely proportional to the length of the bi-metal such that the bi-metal must be of sufficient length to provide adequate tripping force. It has been determined, that the distance that the end of the bi-metal becomes displaced is proportional to the second power of the bi-metal length. Other factors that affect the bi-metal displacement and tripping force such as composition and shape must also be taken into consideration in the bi-metal trip unit design. With elongated bi-metals such as described within the aforementioned U.S. Pat. No. 4,679,016, additional space within the circuit breaker housing must be provided to allow for the large transfer distance between the circuit breaker trip bar and the end of the bi-metal. When the ampere-rating of the circuit breaker is increased, a heavier bi-metal trip unit is employed because of the higher currents passing through the bi-metal at the higher ampere-rated circuits. It would be economically beneficial to be able to linearize the bi-metal whereby the transfer distance of the end of the bi-metal varies in direct proportion to the bi-metal length while allowing a single bi-metal geometry to be used over a circle range of ampere-ratings without damage due to overheating at the higher ratings or lack of trip response at the lower ratings.
Accordingly, one purpose of the invention is to provide a thermal and magnetic trip unit wherein the transfer distance of the bi-metal is linearly proportional to the length of the bi-metal while preventing the bi-metal from overheating at higher ampere loadings.